This invention relates to a method and system for simultaneously broadcasting and receiving digital and analog (e.g., FM) signals in a multipath environment.
In recent years, the quality of commercial audio broadcast signals as delivered by radio transmitters through atmospheric free-space has been eclipsed by the quality of stored program material, such as digital compact disc and audio tape technology. The quality differential of such stored digital program material over conventional analog frequency modulated (FM) broadcasting is so significant that there has been a market shift in listener preference to the stored digital program material. Further adding to this market shift is the increased degradation of FM signal quality, particularly in highly urban areas, due to multipath and noise.
Signals with line-of-sight propagation are subject to interference and fading from reflected copies of the signal, both narrowband and wideband. Such interference resulting from the simultaneous reception via multiple propagation paths between the transmitter and receiver is commonly referred to as multipath (MP), the different propagation paths having varying times-of-arrival, amplitude, and phase.
One of the most difficult environments in which to achieve high quality digital radio communication is the mobile reception of atmospheric free space signals in urban areas. The principal impairment in such environments arises from multipath. Tall buildings and the like act as strong MP reflectors, particularly in the very high frequency (VHF) region of from about 30-300 MHz.
The adverse effects of multipath (MP) on an isolated signal waveform may be grouped at least into the following three categories: fading, dispersion, and intersymbol interference. Fading involves rapid amplitude variation as propagation paths constructively and destructively interfere, but may be controlled under certain circumstances with automatic gain control (AGC) circuits.
Dispersion is caused by time-varying phase disruption within and between bauds (symbols), and may be controlled under certain circumstances with an automatic equalizer.
Intersymbol interference (ISI) is caused by the interaction of one symbol (or waveform) with other symbols in time. An automatic equalizer, which may be used to correct phase dispersion, may compensate for adverse effects of intersymbol interference when the symbol shape of the interfering waveform is approximately identical to that of the desired channel. However, it is difficult and expensive to correct intersymbol interference from undesired propagation paths which represent different symbols of bit patterns that substantially precede or succeed the desired symbol in time.
Therefore, a conventional correction technique is to increase the time duration of the symbol interval (or baud interval) to be much longer than the expected multipath delay. Typical expected multipath delays generally range from a maximum of about 5 to about 30 microseconds (.mu.s) for a VHF channel. However, the increase in time of the baud or symbol interval leads to a decreased data rate.
In multichannel systems with increased baud intervals, the shapes and characteristics of the basis waveforms have significant influences on the BER. (Basis waveforms are the unmodulated sequences representing the data carrier of each channel.) Accordingly, there has been research into desirable characteristics of signal waveforms usable in such multichannel environments for producing superior performance. This research has often been conducted in combination with the use of conventional correlation receivers. In a conventional correlation receiver, a satisfactory received signal is one which satisfies the spectral confinement requirements of the particular application, and is characterized by predetermined cross-correlation and autocorrelation properties.
The cross-correlation property (or orthogonality) is measured between a single signal waveform in a set and all other members of the waveform set. Low cross-correlation is important in multichannel carrier systems in order to ensure that the individual carriers may be recovered and recognized independent of one another. The cross-correlation represents the degree to which a particular waveform is mathematically correlated with one or more other waveform in the set. The smaller the absolute value of the cross-correlation between any two waveforms, the more unique are the waveforms in the correlation sense. Therefore, an ideal signal set for a correlation receiver has a cross-correlation of close to about zero at the sampling point among all pairs of the set. (In other words, it is a set where the waveforms are mutually orthogonal.) Good cross-correlation properties are also required for satisfactory channel performance absent multipath because channels act as sources of interference to each other.
Good autocorrelation is of primary importance in multipath environments because reception requires distinguishing among similar signals with varying times of arrival. (Autocorrelation is a measure of how unique a signal is when compared to itself in a correlation receiver when shifted in time by a positive or negative amount of time shift. An ideal signal set with respect to autocorrelation is one where the autocorrelation for each signal is at a minimum (or has a low value) for substantially all positive and negative time shifts and is at a maximum for about zero offset or, in other words, for relatively no time shift at all.)
Signal waveforms constructed from amplitude samples of unconstrained (or unshaped) and non-orthogonal noise sequences have been proposed and utilized in prior art communication systems (e.g., spread spectrum applications). In a similar manner, prior art systems have utilized prime polynomials to generate pseudo-random binary sequences (also known as PN or direct sequence) which are limited to the values +1 and -1. Such bi-valued systems possess noise-like properties to a limited extent.
The prior art method of Code Division Multiple Access (CDMA) utilizes long baud intervals in a plurality of digital data channels, each carrier being a binary sequence obtained from, for example, Gold codes or Rademacher-Walsh codes. CDMA systems are spread spectrum systems that use multiple binary-valued codes to achieve a higher throughput or increased capacity than a single spread spectrum code. CDMA codes generally must make a tradeoff between cross-correlation and autocorrelation, but typically cannot satisfy acceptable characteristics with respect to both.
A primary disadvantage of CDMA is that it does not permit spectral shaping of the carrier(s) without significant destruction of the sequence properties. Additionally, the number of different acceptable signals which may be generated by CDMA codes is limited by the bi-valued nature of such signals.
In applications where spectrum compliance is not an issue, direct-sequence spread spectrum techniques which utilize noise-like waveforms are effective in combating multipath. However, existing techniques for constructing noise (or the more restrictive example of pseudo-noise) waveforms do not permit arbitrary constraints in the shape of their spectral response without significantly disrupting the resulting waveform properties. This is important because practical systems require band limiting filters or similar processing in order to stay within a fixed frequency allocation and/or reject particular narrowband interference. Furthermore, although the cross-correlation is small in spread spectrum systems, it is generally non-zero and hence the signal waveforms act as interferers to one another even in the absence of multipath.
U.S. Pat. No. 5,278,826 discloses a method and apparatus for digital audio broadcasting and reception wherein a system is provided for transmitting and receiving through free space a composite signal consisting of a frequency modulated (FM) analog signal and a multicarrier modulated digital signal which is especially adapted to be resistant to multipath degradation. The FM signal and digital multicarrier modulated signal are fully coherent. The digital signal comprises a plurality of carriers having a maximum amplitude at least 20 dB below the unmodulated FM signal, preferably 30 dB below the FM signal. Unfortunately, the multicarriers making up the digital signal in this patent are narrowband in nature, each carrier or channel being a single tone which is phase modulated. A problem with such carriers is that multipath (MP) is a frequency selective phenomenon which alters or destroys some frequencies while letting others alone. Thus, narrowband carriers are extremely vulnerable to the adverse effects of multipath. Furthermore, the digital frequency spectrum in this patent is extremely close to the FM center frequency, thus resulting in interference between the FM and digital signals.
U.S. Pat. No. 4,403,331 discloses a method and apparatus for transmitting digital data over limited bandwidth channels, with a set of waveforms being mutually orthogonal to one another and bi-phase data modulation in order to use a correlation-type multiple channel or multicarrier receiver. This patent discloses a technique for determining eigenvectors for the basis functions which maximize the spectral occupancy of the carrier waveforms primarily by utilizing a longer baud interval. The basis functions are based on a fixed number of sinusoids (which are not noise-like), and the system utilizes optimization in the frequency domain. Unfortunately, this does not translate into good autocorrelation properties or result in waveforms which may be made phase-continuous at the baud boundaries. The lack of phase continuity at baud boundaries increases intersymbol interference, thereby limiting the ability to properly receive signals with good BER. Optimization in the frequency domain does not translate necessarily into optimization in the time domain.